This invention relates to a method and arrangement in tail threading in a paper machine, in which the paper web is dried in production by means of a plurality of dryer groups forming a dryer section, and in which, in the tail threading, the threading tail is cut, taken as wetted to the finishing device and spread to its full width for production.
Web breaks in a paper machine lead to a need to rapidly restart production. If production is started by taking the tail end of the web from the drier section to the finishing device, tail threading problems arise, particularly in overdried newsprint, LWC- and SC-paper machines, where the moisture content of the paper is 2.5%-4.0%. The problems are caused by the manufacturing process requirements for the paper grades in question. The web is overdried, to allow it to be profiled to an even quality. Overdrying causes shrinking tensions in the CD-direction i.e. cross direction tension to the web.
When using machines with a single-fabric dryer section, the web is supported firmly against the drying fabric, by means of vacuum rolls and blow-boxes. This gives no opportunity for the release of the cross-direction tension in the web. Thus, the inelasticity of the web caused by the unsuitable moisture level often results in the web splitting in the cross or long direction when it is being cut and spread, thus lengthening the duration of the break.
Nowadays, to permit successful tail threading, the moisture percentage of the paper is increased during tail threading, by reducing the steam pressure in the dryer cylinders. This creates the flexibility and elasticity in the paper, which are required for successful tail threading. Once the web has been successfully spread onto the finishing device, the steam pressures in the dryer cylinders are increased to their production values. The cooling of the dryer cylinders and the return of their pressures to production levels make this an operation that creates further delays, resulting in a great deal of bottom-end and break rejects and a loss of production time.
Document FI-104337 discloses a wetting device in which, with the threading tail covered, the wetting device wets the threading tail. The position of the wetting device is described as being the end of the dryer section, in which it located is after the cutting point. Though the wetting of the tail will enhance its strength in the tail threading, it will not eliminate the problem of bursting, which will appear at the spreading phase.
Another document describing the prior art is FI-88813. This describes a solution which wets only the cutting trail of the threading tail in the case of bias-cutting. The device is situated before the cutting blade.
Further, the document U.S.-5862608 is known, which describes a solution based on the cooling of the dryer cylinders and through that achieves an increase in the moisture content of the paper web. The dryer cylinders are cooled by wetting the dryer wires. Every dryer group must have own device. The described solution has a certain delay, before there is an effect on the web moisture.
The present invention provides a method and arrangement by means of which the moisture content of the paper can be increased to a suitable level (4-8% , preferably 5-6%) essentially over the whole width of the web while threading the tail and spreading it, so that the equipment in the dryer section can be kept operating continuously during production, at the values required by production.
More specifically, the method in tail threading in a paper machine, in which the paper web is dried in production by means of a plurality of dryer groups consisting of a dryer section, and in which tail threading the threading tail is cut, taken as wetted to the finishing device, and spread to its full width for production, and in which method, before the threading tail is taken to the finishing device, the dryer section is kept in the settings of production operation or is set to otherwise correspond to production, except that a wetting process is started, in which the web is wetted at the full width for the duration of the tail threading, essentially before its cutting, to achieve an even moisturizing effect, and the wetting process is stopped after web is being spread.
An arrangement in a paper machine, which includes a multi-stage dryer section, threading tail cutting devices, a finishing device, and devices to take the threading tail to the finishing device, and possible rewetting devices, is characterized in that the arrangement includes full width wetting devices on the middle of the dryer section before cutting devices and which are arranged to wet the web, essentially over its full width, for the duration of the tail-threading phase.
The spraying of water onto the paper is a considerably more immediate measure than the previous method, because the web is at once either moister when the spray valve is open or drier when the spray valve is closed. When the wetting-spray is opened during a break, bottom-end and/or break rejects during the delay in the change in the steam pressure in the dryer-cylinder are entirely eliminated. If the wetting-spray is dimensioned according to the operation speed of the machine and the grammage, a wetting window can be accepted, in which the same wetting creates a moisture content of 8% in smaller grammages and a moisture content of 6% in larger grammages, at the same machine speed.
In the following, the invention is disclosed in detail, with reference to the accompanying drawings, which show a paper machine and the location of the wetting device in it.
These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings.